1. Field of the Invention
The invention relates to a process for continuously preparing (N-organylaminoorganyl)- and (N,N-diorganylaminoorganyl)triorganylsilanes from corresponding triorganylsilylorganyl halides and N-organyl- or N,N-diorganylamines.
2. Description of the Related Art
The prior art discloses various processes for preparing (N-organylaminoorganyl)- and (N,N-diorganylaminoorganyl)triorganylsilanes. Among the processes described to date, the reaction of (haloorganyl)silanes with corresponding amines has been found to be by far the most favorable with regard to process technology and economic aspects.
What is advantageous in this case is in particular the high availability of (chloroalkyl)silanes, which are obtainable by means of photochlorination of alkyl-silanes or hydrosilylation of corresponding halogen-substituted olefins onto Si—H-containing compounds, and find use, for example, as intermediates for the synthesis of a multitude of organofunctional silanes. Moreover, it is possible in this process to employ a large number of readily available primary and secondary amines to form the (N-organylaminoorganyl)- and (N,N-diorganylaminoorganyl) triorganylsilanes, which enables a very wide field of use of the process and, as a result, inexpensive product change on existing industrial manufacturing plants.
GB 686,068 A discloses (amino)-, (N-organylamino)- and (N,N-diorganylaminomethyl)- or (N,N-diorganylaminoethyl)triorganylsilanes. Moreover, GB 686,068 A describes a process for reacting corresponding (chloromethyl)- or (bromomethyl)triorganosilanes with ammonia, a primary or secondary amine at temperatures of at least 50° C. to prepare the (aminoorganyl)-, (N-organylaminoorganyl)- and (N,N-diorganylaminoorganyl)triorganylsilanes. In general, the (chloromethyl)- or (bromomethyl)triorganosilanes were initially charged in a flask or autoclave depending on the boiling point of the amine compounds used, and heated to temperatures above 100° C., preferably 110-130° C. In the case of relatively high-boiling amines (e.g. cyclohexylamine), the mixing sequence was reversed and the (chloromethyl)- or (bromomethyl)-triorganosilanes were added to the heated amine. The reaction time, depending on the amine compound to be converted, was from 2 to 8 hours.
The (aminomethyl)silane derivatives are prepared by the process described in DE 1812564 A1, by reacting a (chloromethyl)- or (bromomethyl)silane derivative with ammonia or a primary amine. The reaction is effected at temperatures of 80 or 100° C. within a period of 3 or 2 hours, the amine already having been initially charged completely at the start of the reaction in a molar excess of 1:3.2-6.
The processes described in GB 686068 A and DE 1812564 A1 have comparatively long reaction times of several hours. The achievable yields are nevertheless low, which is a consequence of the long reaction times and the associated increased formation of by-products, among other factors (on this subject, see below). Moreover, the products are not obtained in the required purity and have to be purified in a complicated manner before their further use. For example, the products obtained by the process described contain large amounts of ionic chloride or bromide. This limits their industrial use without purification, for example for use in sealants to be applied to metallic surfaces, the ionic content results in promotion of corrosion, among other reasons.
The chloride- or bromide-containing impurities which occur here are in particular the hydrochlorides or hydrobromides of the amines used in the synthesis or the hydrochlorides or hydrobromides of the target compounds.
In this context, it has been observed that mixtures of (aminomethyl)silanes and such hydrochlorides or -bromides can lead, at the relatively high temperatures necessary during preparation and distillative purification of the target compounds, to an undesired and exothermic decomposition of the target compounds with cleavage of the Si—C bond and simultaneous formation of corresponding N-methylated amines. The N-methylamines thus formed influence the course of the process in a very undesired manner. This effect appears to correlate with the basicity of the amine compound to the extent that such decomposition reactions are favored with decreasing basicity of the corresponding (aminomethyl)silane. For this reason, a low halide content of the (aminomethyl)silanes is also necessary from a safety point of view.
The prior art discloses processes for reducing halide contents in alkoxysilanes, for example those which are based on the precipitation of the dissolved halide by adding alkalimetal alkoxide or alkaline earth metal alkoxide salts (for example EP 0702017 A1, DE 69306288 T2, DE 19513976 A1), but superstoichiometric amounts of the salts are needed in this case for the simple and efficient reduction of the halide content, since merely stoichiometric amounts cannot achieve the desired complete halide removal. All (aminomethyl)silanes investigated to date have an unfavorable tendency under these conditions, e.g. the simultaneous presence of free alcohols and strong bases, to participate in decomposition reactions. An alternative process which should enable reductions of chloride contents in alkoxysilanes by introduction of ammonia is described in DE 19941283 A1, but the possibility of using (aminoalkyl)alkoxysilanes is explicitly ruled out in this process.
Moreover, it was observed that the decomposition of (aminomethyl)silanes previously mentioned takes place not only in the presence of hydrochlorides or alcohols and bases, but rather, in particular, even the sole presence of alcohols may be sufficient to bring about the undesired formation of corresponding N-methylamines. The N-methylated amine derivatives thus formed compete during the reaction with unmethylated amine still present for reaction with the (halomethyl)silane, and lead finally lead to the formation of (N-methylaminomethyl)silanes which cannot be removed again by distillation from the target compounds. To avoid this undesired side reaction, it is therefore necessary to ensure the absence of alcohols in the reaction mixture. When at least one of the organyl radicals on the silicon atom is an alkoxy group, it is possible to use only amines with low water content in the process, since alcohols can otherwise be released by preceding reaction of the silane with water present in the amine.
Moreover, the heating of the completely premixed solution of silane and amine described in DE 1812564 A1 is of concern for an industrial scale reaction for safety reasons owing to the exothermic reaction of the two components.
DE 10353063 discloses an improved process for preparing (N-organylaminoorganyl)- and (N,N-diorganylaminoorganyl)triorganylsilanes, in which the starting silane is initially charged and heated and then the corresponding amine is added continuously.
Moreover, the halide contents in the target compounds can be reduced by adding nonpolar solvents to the crude mixtures and then removing the precipitated salts. The use of low-water content amines prevents the release of alcohols during the synthesis, and hence also the formation of by-products. Surprisingly, it was also possible to remove the amine hydrohalide or (aminomethyl)silane hydrohalide from the mixture during the process by salt exchange with ammonia, which also prevents the formation of the N-methylated by-products. It was surprisingly possible to obtain chloride-free (aminomethyl)silanes preferentially by introducing ammonia into the crude product or isolated end product, even one though skilled in the art would expect the opposite from the teaching of DE 19941283 A1. The improved process described in DE 10353063 was, however, performed in batch operation and thus did not offer the desired increase in space-time yield and product quality.